1. Field of the Invention
The present invention relates to a switching circuit of a BS (Broadcast of Satellite) converter for selecting between reception signal bands of satellite broadcast.
2. Description of the Prior Art
According to digitization of a BS system and the corresponding increase in number of channels, there has been an attempt at widening each reception frequency band. For instance, in a reception frequency band of 10.7 GHz to 12.75 GHz, the frequency band is generally divided into a frequency band of 10.7 GHz to 11.7 GHz and a frequency band of 11.7 GHz to 12.75 GHz. Consequently, in order to receive both the frequency bands, there are necessitated two independent sets of an antenna and a BS converter.
In recent years, in order to receive both the frequency bands by means of one antenna and one BS converter, there has been desired a system capable of receiving both the frequency bands by means of one BS converter by changing an oscillation frequency of a local oscillator provided in a BS converter based on an external pulse signal superimposed on a power voltage.
As a pulse signal processing circuit, a digital-to-analog converter circuit such as a peak detection circuit and a pulse-to-voltage converter circuit (pulse rate meter) are known.
FIG. 4 is a circuit diagram of the pulse rate meter. The pulse rate meter is a circuit for obtaining an output signal proportional to a pulse rate (representing an amount of pulses included in a second) of an input signal.
In the circuit shown in FIG. 4, when a pulse having a peak value Vs is incorporated into the input signal, an electric charge Q (=Vs.multidot.Cp) flows into a capacitor Cf having a capacitance Cf through a pump circuit composed of diodes D1 and D2, and there can be obtained an output signal of a staircase having a good linearity wherein a peak value of one step corresponds to Vo (=(Cp/Cf).multidot.Vs). In the above place, in order to reduce a swing range of a meter M, the Cf/Cp value is greatly increased and the Rf value also is increased.
However, when the above-mentioned pulse rate meter is used as a pulse processing circuit for a switching circuit of a BS converter, there are problems as follows.
That is, in the pulse rate meter, an analog output corresponding to a pulse level, pulse width or the like can be obtained. However, a required circuit characteristic of a pulse processing circuit for a switching circuit of a BS converter as described above is to decide only whether a pulse is present or absent. Therefore, a complicated circuit as shown in FIG. 4 is not necessitated.
Furthermore, the pulse voltage to be superimposed on the power voltage is a very small voltage of 0.6 V.+-.0.2 V, and since a very great Cf/Cp rate is set in a pulse rate meter as shown in FIG. 4, a significantly degraded sensitivity results when the pulse peak value Vs of the input signal is too small. Therefore, in a pulse rate meter as shown in FIG. 4, it is very difficult to make the BS converter perform a switching operation.